1. Field of the Invention
This invention relates to a method for the dehydrogenation treatment of a hydrogenated amorphous silicon film and to a method for forming a crystalline silicon film.
2. Description of the Related Art
Conventionally, the driving circuit of a display device such as a liquid crystal display device is formed of an amorphous semiconductor film formed on a glass substrate. Due to the increasing expansion of IT market, the information to be handled is now digitalized and accelerated in processing speed, so that the display device is also required to display high-quality pictures. In order to meet such a requirement, there has been developed a driving circuit wherein a switching transistor for switching each of pixels, for example, is formed on a crystalline semiconductor film region such as a crystalline silicon film, thereby making it possible to realize the acceleration of processing speed and to enhance the quality of pictures.
The crystalline silicon film can be obtained through the crystallization of amorphous silicon film that has been formed on a glass substrate. As the method of crystallizing the amorphous silicon film, a method of melting the amorphous silicon film by the irradiation of microplasma jet is proposed in JP 2006-60130 (KOKAI).
In JP 2007-73941 (KOKAI), the employment of excimer laser annealing (ELA) method is proposed as a method of crystallizing the amorphous silicon film. In this case, by taking into consideration the resistance of amorphous silicon film to laser, a hydrogenated amorphous silicon film that has been undergone dehydrogenation treatment is employed as the amorphous silicon film. This hydrogenated amorphous silicon film is generally formed by a plasma enhanced chemical vapor deposition (PECVD) method.
For example, an amorphous silicon film which has been formed by the PECVD method using silane gas as a raw material contains about 10% to 15% of bonded hydrogen. Such an amorphous silicon film has sufficient density and can be formed practical speed. However, the bonded hydrogen may become a cause for the ablation of film on the occasion of performing laser crystallization. In order to eliminate bonded hydrogen from the film, there has been conventionally employed a process wherein the hydrogenated amorphous silicon film that has been formed by the PECVD method is heated for several hours at a temperature of 500° C. or more in an electric furnace.
By performing this heat treatment, the content of hydrogen in the film is reduced to 1% or less, thereby making it possible to achieve the laser crystallization without generating the ablation. However, when a glass substrate which is low in heat resistance is employed, the heat treatment at high temperatures would become a cause for generating deformation and moreover the heat treatment for several hours may lead to the deterioration of throughput.
In order to obtain a crystallized silicon film of high qualities from a hydrogen-containing amorphous silicon film which has been formed on a non-heat-resistant substrate such as a glass substrate, it is required to perform the heat treatment without deforming and degrading such a substrate. However, no one has succeeded in discovering an appropriate method for overcoming this problem to date.